Antenna range for providing a plane
x		wave for antenna measurements



FIP8102 XR 3,302,205

1 AflENNA FiANGE FOR PROVIDING A PLANE 5 WAVE FOR ANTENNA MEASUREMENTS Filed March 13, 1964 5 Sheets-Sheet 1 INVENTOR.

Richard C. Johnson ATTO RNE Y5 Jan. 31, 1967 File d March 13, 1964 R. C. JOHNSON ANTENNA RAN WAVE FOR 3,302,205 GE FOR PROVIDING A PLANE ANTENNA MEASUREMENTS 5 Sheets-Sheet 1.

INVENTOR.

Richard C. Johnson ATT RNE YS Jan. 31, 1967 Q JOHNSQN 3,302,205

ANTENNA RANGE FOR PROVIDING A PLANE WAVE FOR ANTENNA MEASUREMENTS Filed March 13, 1964 5 Sheets-Sheet .3

INVENTOR.

Rlchard C. Johnson AT TORNE '5 5 Sheets-Sheet 4 J n- 9 R. c. JOHNSON ANTENNA RANGE FOR PROVIDING A PLANE WAVE FOR ANI'LNNA MEASUREMENTS Filed March 13, 1964 Fig. 4

m ENTOR Richard C. Johnson A TTORN E Y6 Jan. 31. 1967 c JQHNSON 3,302,205

ANTENNA RANGE FOR PROVIDING A PLANE 6 Filed March 13, 1964 WAVE FOR ANTENNA MEASUREMENTS 5 Sheets-Sheet INVENTOR. Richard C. Johnson A TTORNEYS United States Patent 3,302,205 ANTENNA RANGE FOR PROVIDING A PLANE WAVE FOR ANTENNA MEASUREMENTS Richard C. Johnson, Atlanta, Ga., assignor to Georgia Tech Research Institute, Atlanta, Ga., a corporation of Georgia Filed Mar. 13, 1964, Ser. No. 351,735 4 Claims. (Cl. 343-703) This invention relates to apparatus for and a method of making antenna measurements and more particularly, to an antenna range for and a method of providing a plane wave for antenna measurements which is uniform and free of distortion and which is emitted at only a relatively short distance from a test antenna.

It is customary to make antenna measurements related to the radiation or other characteristics of a test antenna by measuring the output from the test antenna when a plane wave is incident upon the test antenna. The arrangement of apparatus necessary to obtain a plane wave for antenna measurements is generally referred to as an antenna range and the accuracy with which an antenna range permits antenna measurements to be made is de pendent upon the antenna range providing a wave front incident upon the antenna range which is a plane wave substantially uniform and free of distortion.

Previous antenna ranges have generally provided a wave front which only approximates a plane wave. This is because most previous antenna ranges have used an illuminating antenna to emit a spherical wave front and have obtained an approximation of a plane wave by positioning the test antenna sufiiciently far from the illuminating antenna for that portion of the spherical wave front incident upon the test antenna to resemble a plane wave.

A further difficulty with these previous antenna ranges using a spherical wave front is that relatively large distances between the illuminating antenna and a test antenna are frequently necessary in order for that portion of a spherical wave front incident upon the test antenna even to closely approximate a plane wave. These distances make many of these previous antenna ranges impractical for use within a building which would shelter equipment and personnel from weather and other environmental conditions so as to make the obtaining of antenna measurements convenient and substantially independent of weather and environmental conditions.

Moreover, these previous antenna ranges using a spherical wave front are characterized by reflection of the spherical wave front from surfaces adjacent to the antenna range. This reflection of the spherical wave front distorts the wave front incident upon a test antenna so as to reduce the accuracy of an antenna range under almost all conditions and if the antenna range is placed within a building, the reflection of the spherical wave front from walls and objects and the resulting distortion of the wave front incident upon the test antenna become so great as to render the antenna range substantially useless for making antenna measurements. This can be avoided only by using especially shaped rooms and elabo- -rate and expensive arrangements of absorbing material.

In addition, these previous antenna ranges using a spherical wave front provide no convenient means for defocusing or varying the wave front incident upon a test antenna so that measurements related to the focusing of the test antenna can be readily obtained.

Previous efforts have been made to provide an antenna range in which the wave front incident upon a test antenna is not simply an approximation of a plane wave and in which substantial distances between a test antenna and an illuminating antenna or other illuminating mem- "ice ber are not necessary. These previous efforts have resulted in antenna ranges using lenses or parabolic reflectors to form a plane wave which is directed by the lens or parabolic reflector toward the test antenna. These antenna ranges do not require a substantial distance between a test antenna and the illuminating member of the antenna range and they have generally permitted that defocusing or controlled distortion of the plane wave incident upon a test antenna necessary for focusing adjustments of the test antenna to be readily made.

The difliculty with these previous antenna ranges using lenses and parabolic reflectors is that they have not provided a plane wave which is free from distortion. This is because these previous antenna ranges have been char acterized by random and uncontrolled reflections of the wave front and by induced currents and other phenomena within the antenna range which distort the plane wave incident upon the test antenna.

The antenna range and method of determining the characteristics of a test antenna disclosed herein provide a plane wave free of distortion and permit antenna measurements to be accurately made with only a relatively short distance between the test antenna and the illuminating member of the antenna range. This is because the invention disclosed herein provides a substantially undistorted plane wave only a relatively short distance from the illuminating member of the antenna range.

The antenna range disclosed herein is equally well adapted to use within or without a building and when used within a building, the antenna range permits antenna measurements to be made without interference from in clement weather or other environmental factors. Moreover, especially shaped rooms and elaborate arrangements of absorbing material are unnecessary for use of the antenna range within a room. Whether used within or without a building, the antenna range permits antenna measurements to be made with only a relatively short distance between a test antenna and the illuminating member of the antenna range.

These improvements in antenna ranges are provided by an illuminating member which comprises an emitting member from which a wave front is emitted and a reflecting member which focuses the wave front from the emitting member to form a plane wave. The emitting member is positioned with respect to the reflecting member so that the plane wave passing from thereflecting member to a test antenna is not reflected from the emitting member or in any way distorted by the emitting member.

In addition, the reflecting surface of the reflecting member is sufliciently large relative to that portion of the reflecting surface upon which the wave front from the emitting member is incident to minimize edge currents and other phenomena which would distort the plane wave. Thus, the antenna range provided by the apparatus and method of the invention creates a wave front which is a substantially undistorted plane wave as the wave front leaves the reflecting member. This permits a test antenna to be positioned relatively close to the illuminating member with a plane wave incident upon it.

These and other features and advantages of the present invention will be more clearly understood from the following detailed description and the accompanying drawings in which like characters of reference designate corresponding parts throughout and in which:

FIG. 1 is a perspective view of an embodiment of the antenna range disclosed herein.

FIG. 2 is a cross-sectional view of the antenna range shown in FIG. 1 taken in a vertical plane containing the center line of the parabola reflector of the illuminating member.

FIG. 3 is a perspective view of a second embodiment of the antenna range disclosed herein.

FIG. 4 is a cries-sectional view of the antenna range shown in FIG. 3 taken in a vertical plane containing the center line of the parabola reflector of the test antenna.

FIG. 5 is a cut-away view of the horizontal reflector of the reflecting member shown in FIG. 3.

FIG. 6 is an enlarged cross-sectional view of that portion of the horizontal reflector shown in FIG. 5 taken on line 66 in FIG. 5.

These figures and the following detailed description disclose specific embodiments of the invention, but the invention is not limited to the details disclosed since it may be embodied in other equivalent forms.

The invention disclosed herein is best understood in terms of the apparatus of the invention in which the illuminating member M comprises an emitting member 10 which emits a wave front which is not a plane wave and a reflecting member 11 which reflects the wave front emitted by the emitting member 10 so as to form a plane wave. In that embodiment of the antenna range shown in FIG. 1 and in FIG. 2, the emitting member 10 is a horn radiator 12 having a bracket 19 slidably positioned on a horizontal arm 13 attached to a collar 14 which is slidably carried by a standard 15. The horn radiator 12 is of a known type having minimum back radiation and which is capable of emitting electromagnetic energy with a unidirectional pattern and with a wave front which resembles a portion of a spherical wave. The horn radiator 12 will be understood to have a focal point and is fed in known manner through a wave guide 30 from any convenient known source of electromagnetic energy at selected frequencies (not shown).

The reflecting member 11 of the antenna range shown in FIG. 1 and in FIG. 2 is a rotational parabola reflector 16 supported in known manner by a frame 17 with its axis of rotation substantially horizontal. The parabola reflector 16 is of known type which reflects a spherical wave incident upon its reflecting surface 18 as a plane wave when the spherical wave is emitted from the focal point of the reflecting surface 18. It will be understood that when the focal point of the horn radiator 12 coincides with the focal point of the reflecting surface 18 of the parabola reflector 16, the wave front emitted from the horn radiator 12 will be reflected from the reflecting surface 18 of the parabola reflector 16 as a plane wave. The focal point of the horn radiator 12 is positioned to coincide with that of the reflecting surface 18 of the parabola reflector 16 by slidably moving the bracket 19 along the length of the arm 13 and the collar 14 along the length of the standard 15. Means of known type (not shown) is provided for fixedly positioning the bracket 19 at a selected position along the length of the arm 13 and means of known type (not shown) is provided for fixedly positiom'ng the collar 14 along the length of the standard 15.

The horn radiator 12 is positioned on the bracket 19 so that the entire wave front emitted by the horn radiator 12 strikes the reflecting surface 18 of the parabola reflector 16 above a horizontal plane of reference which is in turn above the horn radiator 12. It will be understood that with the axis of rotation of the rotational parabola reflector 16 substantially horizontal, this wave front is reflected from the reflecting surface 18 along a path substantially horizontal. The result is that the plane wave from the reflecting surface 18 of the parabola reflector 16 passes entirely above the horn radiator 12. This prevents distortion of the plane wave by reflections of the plane wave from the horn reflector 12, the arm 13 or the standard 15.

The horn reflector 12 is also positioned and the size of the parabola reflector 16 is selected so that the outer edge 20 of the reflecting surface 18 is relatively remote from that portion 21 of the reflecting surface 18 of the parabola reflector 16 upon which the wave front from the horn radiator 12 is incident. It will be understood by those skilled in the art that this positioning of the horn radiator 12 and the size selected for the parabola reflector 16 serve to prevent edge currents from being induced which would result in distortion of the plane wave reflected: from the parabola reflector 16. This is because those skilled in the art will understand that edge currents which would distort the wave front reflected from the reflecting surface 18 will be induced only if electromagnetic energy is incident on the reflecting surface 18 at or adjacent to the outer edge 20 0f the reflecting surface 18.

It will now be understoodthat the emitting member 10 and the reflecting member 11 are arranged in that embodiment of the antenna range shown in FIG. 1 and FIG. 2 to provide a plane wave which is not distorted by reflections of the wave front from the emitting member 10 or by edge currents induced in the reflecting member 11. It will also be understood that when a test antenna A is mounted on a suitable support member 22 and is positioned in known manner so that the plane wave from the reflecting member 11 is incident upon it, antenna measurements may be made in the known and understood manner.

The plane wave reflected from the parabola reflector 16 will be understood by those skilled in the art to be highly directional and it has been found that if reflecting surfaces (not shown) are not placed adjacent to the antenna range so that the plane wave is reflected from them between the parabola reflector 16 and the test antenna A, the plane wave incident upon the test antenna A is substantially uniform and free of distortion. Thus, the antenna range of the invention provides a wave front which is reflected from the parabola reflector 16 as a substantially uniform and undistorted plane wave and which is not inherently or easily distorted between the parabola reflector 16 and a test antenna A.

This not only permits highly accurate antenna measurrnents to be made, but it also permits a test antenna A to be positioned relatively close to the parabola reflector 16 provided that care is exercised to insure that the position of the test antenna A does not result in the wave front emitted by the emitting member 10 striking the test antenna A or the support member 22 before it strikes the reflecting member 16. Thus, the antenna range of the invention requires only a relatively small amount of space and is ideally suited to the obtaining of antenna measurements within a building.

Moreover, by slidably moving the bracket 19 on the arm 13 and the collar 14 on the standard 15, the focal point of the horn radiator 12 may be conveniently moved so that it no longer coincides with the focal point of the reflecting surface 18 of the parabola reflector 16. Such motion of the focal point of the horn radiator 12 will be understood to cause distortion of the wave front reflected from the parabola reflector 16 and those skilled in the art will recognize that the amount of distortion of the wave front reflected from the parabola reflector 16 is dependent upon the amount by which the focal point of the horn radiator 12 is displaced from the focal point of the reflecting surface 18 of the parabola reflector 16.

Those skilled in the art will also recognize that by distorting the plane wave incident upon the test antenna A, antenna measurements are obtained which aid in the easy and convenient focusing of the test antenna A. Thus, the antenna range of the invention is well suited to obtaining those antenna measurements necessary for focusing a test antenna A.

In that specific embodiment of the antenna range disclosed herein shown in FIG. 3 and FIG. 4, the reflecting member 11' of the illuminating member M comprises a horizontal reflector 23 and a vertical reflector 24. The horizontal reflector 23 is formed by an upper conducting plate 25, a lower conducting plate 26, and a reflector insert 27.

The conducting plates 25 and 26 are substantially rectangular and are spaced apart parallel to each other by spacers 28 extending between the corners of the upper conducting plate 25 and the corners of the lower conducting plate 26. The reflector insert 27 extends between the upper conducting plate 25 and lower conducting plate 26.

The vertigl reflector 24 is a cylindrical parabolic re flector having a reflecting surface 32 curved in a parabolic manner about a horizontal axis. The vertical reflector 24 is supported by a frame 17 and the horizontal reflector 23 is positioned by a frame 31 so that the conducting plates 25 and 26 are inclined upwardly toward the reflecting surface 32 of the vertical reflector 24 and with their uppermost edges 33 and 34 horizontal and parallel to the reflecting surface 32 of the vertical reflector 24.

The emitting member of that embodiment of the antenna range shown in FIG. 3 and FIG. 4 is a horn radiator 12 positioned between the conducting plates and 26 adjacent one corner of the conducting plates 25 and 26 at the uppermost edges 33 and 34 of the conducting plates 25 and 26. The reflector insert 27 is positioned between the conducting plates 25 and 26 so that the reflector insert 27 is more remote from the vertical reflector 24 than the horn radiator 12' and the horn radiator 12' is positioned between the conducting plates 25 and 26 so that a wave front emitted by the horn radiator 12' travels in known manner between the conductingplates 25 and 26 and is reflected by the reflecting surface of the reflector insert 27. The horn radiator 12 is fed in known manner through a wave guide 41 from any con venient source of electromagnetic energy (not shown) at selected frequencies.

The reflecting surface 40 of the reflector insert 27 is shaped as a segment of a cylindrical parabolic reflector curved about a vertical axis, and the focal point of the horn radiator 12' coincides with the focal point of the reflecting surface 40. Thus, the reflector insert 27 is positioned between the conducting plates 25 and 26 so that a wave front from the horn radiator 12 is reflected by the reflecting surface 40 and passes from between the conducting plates 25 and 26 at the uppermost edges 33 and 34 of the conducting plates 25 and 26.

The horn radiator 12 emits in known manner a wave front which is a portion of a spherical wave and with minimum back radiation. It will now be understood that when the wave front emitted by the horn radiator 12' is reflected by the reflecting surface 40, the wave front becomes a cylindrical wave front symmetrical about a horizontal line of reference at the upper edges 33 and 34 of the conducitng plates 25 and 26. The position of the conducting plates 25 and 26 relative to the reflecting surface 32 of the horizontal reflector 23 is selected in known manner so that the line of focus of this cylindrical wave coincides with the line of focus of the reflecting surface 32. Thus, it will be understood that when this cylindrical wave front is reflected by the vertical reflector 24, the wave front becomes a plane wave. Thus, the horizontal reflector 23 and the vertical reflector 24 cooperate to provide a wave front which is a plane wave when reflected from the vertical reflector 24.

The horn radiator 12' is selected in known manner so that its radiation pattern is unidirectional and the reflecting surface 40 of the reflector insert 27 is sufficiently long for the wave front from the horn radiator 12' to be reflected only by those portions of the reflecting surface 40, which are remote from the ends of the reflecting surface 40. Similarly, the horizontal reflector 23 is positioned relative to the reflecting surface 32 of the vertical reflector 24 so that that portion 36 of the reflecting surf-ace 32 of the vertical reflector 24 upon which the wave front from the horizontal reflector 23 is incident is remote from the edges of the vertical reflector 24. Thus, as with that embodiment of the antenna range shown in FIG. 1 and FIG. 2, the second embodiment of the antenna range shown in FIG. 3 and FIG. 4 avoids those edge currents which result in distor tion of the plane wave.

Moreover, as with that embodiment of the antenna range shown in FIG. 1 and FIG. 2, the embodiment of the antenna range shown in FIG. 3 and FIG. 4 provides an arrangement of an emitting member 10' and a reflecting member 11' in which the plane wave formed by the illuminating member M' is not reflected from: the emitting member 10' so that distortions of the plane wave result. This is because the vertical reflector 24 is positioned in known manner so that the path of a plane wave from the reflecting surface 32 is substantially horizontal and because that portion 36 of the reflecting surface 32 on which the wave front from the horizontal reflector 23 is incident is above a horizontal plane of reference which is in turn above the horizontal reflector 23. Thus, that embodiment of the antenna range shown in FIG. 3 and in FIG. 4 permits accurate antenna measurements to be made for a test antenna A positioned on a support member 22' in the same manner as such measurements are made with that embodiment of the antenna range shown in FIG. 1 and in FIG. 2.

This second embodiment of the antenna range also permits antenna measurements necessary for focusing a test antenna A to be made by simply varying the position of the horn radiator 12' between the conducting plates 25 and 26. This is because motion of the horn radiator 12' from that position in which its focal point coincides with the focal point of the reflecting surface 40 causes the wave front reflected from the vertical reflector 24 to be distorted from a plane wave so that those antenna measurements necessary for focusing the test antenna A may be obtained.

It has been found that induced currents which distort the plane wave provided by the second embodiment of the antenna range shown in FIG. 3 and FIG. 4 result if the conducting plates 25 and 26 are not electrically continuous with the reflecting surface 40. This condition of electrical continuity is difficult to achieve using welding and other known techniques and in that embodiment of the invention shown in FIG. 3 and in FIG. 4, the reflecting surface 40 is provided by forming the reflector insert 27 as an arcuate channel member 42 which is inserted through arcuate slots 29 in the conducting plates 25 and 26.

The inner surfaces of the arcuate channel member 42 are an upper wall 43 which is in the same plane of reference as the inner surface of the upper conducting plate 25, a lower wall 44 which is in the same plane of reference as the inner surface of the lower conducting plate 26, and the reflecting surface 40 of the reflector insert 27. The channel member 42 is machined or otherwise formed from a single piece of material such as steel and as a result the walls 43 and 44 are electrically continuous with the reflecting surface 40. It will be understood that the walls 43 and 44 act as extensions of the conducting plates 25 and 26 and that the channel member 42 provides a convenient means for avoiding those induced currents at the intersections of the conducting plates 25 and 26 with the reflecting surface 40 which distort the plane wave.

Induced currents which would distort the plane wave are avoided where the upper wall 43 is discontinuous with the inner surface of upper conducting plate 25 and where thelower wall 44 is discontinuous with the inner surface of the lower conducting plate 26 by varying the widths of the walls 43 and 44 along the length of the reflecting surface 40 in known manner so that electromagnetic energy reflected from the reflecting surface 40 and electromagnetic energy incident upon the channel member 42 at the outer edges 50 and 51 of the walls 43 and 44 serve to cancel in known manner those induced currents at the outer edges 50 and 51 which would distort the plane wave. It will be understood by those skilled in the art that these selected widths of the walls 43 and 44 result in a voltage null which does. not induce currents where the Walls 43 and 44 are discontinuous with the inner surfaces of the conducting plates 25 and 26.

The two embodiments described of the antenna range disclosed herein permit antenna measurements for a test antenna A or A to be obtained within or without a building to a high degree of accuracy and are well adapted to the method of th 'ginvention. However, it will be tinderstood that a variety of apparatus arrangements permit the emitting from the focal point of an emitting member of a wave front which is incident upon a reflecting member as the wave front traverses a unidirectional path, the forming of a plane wave by a portion of the reflecting member remote from its edges as the wave front from the emitting member is reflected by the reflecting member, the reflecting of the wave front from the emitting member by the reflecting member along a directional path forming an acute angle with the unidirectional path and which does not traverse the position of the emitting member, and the positioning of a test antenna in the plane wave at any of a plurality of distances from the reflecting member. Regardless of the embodiment of the antenna range used, this method provides accurate antenna measurements to be made with the test antenna relatively close to the illuminating member M and without using especially shaped rooms and elaborate arrangements of absorbing material.

It will be obvious to those skilled in the art that many variations may be made in the embodiments chosen for the purpose of illustrating the present invention without departing from the scope thereof as defined by the appended claims.

What is claimed as my invention is:

1. In an antenna range for antenna measurements, means for emitting a wave front, a first parabolic surface contained within a wave guide, and a second parabolic surface, said means for emitting a wave front, said first parabolic surface, and said second parabolic surface being positioned whereby a wave front emitted by said means for emitting a wave front is reflected from said first parabolic surface to said second parabolic surface and subsequently from said second parabolic surface to an antenna being tested.

2. An antenna range as claimed in claim 1 and including means for preventing distortion of the wave reflected from said first parabolic surface.

3. An antenna range providing a wave front incident} upon an antenna being tested, said antenna being tested having an aperture with a maximum aperture dimension and said antenna range comprising a parabolic reflecting means positioned to place said antenna being tested relatively close to and within the perimeter of said parabolic reflecting means, said parabolic reflecting means having a focal point; and an emitting means for emitting a spherical wave front toward said parabolic reflecting means, said emitting means being positioned at said focal point so as to direct said wave front to a small portion of said parabolic reflecting means which is displaced from any edge of said parabolic reflecting means by a distance sufticient to substantially eliminate edge effects; whereby a plane wave is radiated from said parabolic reflecting means toward said antenna being tested and the aperture of said plane wave is substantially equal to said maximum aperture dimension of said antenna being tested.

4. The antenna range of claim 3 wherein said plane wave is radiated from said parabolic reflecting means along a path and said emitting means is displaced from said path.

References Cited by the Examiner UNITED STATES PATENTS 12/1950 Kienow 343781 5/1952 Sichak 343781 OTHER REFERENCES HERMAN KARL SAALBACH, Primary Examiner. ELI LIEBERMAN, Examiner.

A. R. MORGANSTERN, M. NUSSBAUM,

Assistant Examiners. 

1. IN AN ANTENNA RANGE FOR ANTENNA MEASUREMENTS, MEANS FOR EMITTING A WAVE FRONT, A FIRST PARABOLLIC SURFACE CONTAINED WITHIN A WAVE GUIDE, AND A SECOND PARABOLIC SURFACE, SAID MEANS FOR EMITTING A WAVE FRONT, SAID FIRST PARABOLIC SURFACE, AND SAID SECOND PARABOLIC SURFACE BEING POSITIONED WHEREBY A WAVE FRONT EMITTED BY SAID MEANS FOR EMITTING A WAVE FRONT IS REFLECTED FROM SAID FIRST PARABOLIC SURFACE TO SAID SECOND PARABOLIC SURFACE AND SUBSEQUENTLY FROM SAID SECOND PARABOLIC SURFACE TO AN ANTENNA BEING TESTED.
 3. AN ANTENNA RANGE PROVIDING A WAVE FRONT INCIDENT UPON AN ANTENNA BEING TESTED, SAID ANTENNA BEING TESTED HAVING AN APERTURE WITH A MAXIMUM APERTURE DIMENSION AND SAID ANTENNA RANGE COMPRISING A PARABOLIC REFLECTING MEANS POSITIONED TO PLACE SAID ANTENNA BEING TESTED RELATIVELY CLOSE TO AND WITHIN THE PERIMETER OF SAID PARABOLIC REFLECTING MEANS, SAID PARABOLIC REFLECTING MEANS HAVING A FOCAL POINT; AND AN EMITTING MEANS FOR EMITTING A SPHERICAL WAVE FRONT TOWARD SAID PARABOLIC REFLECTING MEANS, SAID EMITTING MEANS BEING POSITIONED AT SAID FOCAL POINT SO AS TO DIRECT SAID WAVE FRONT TO A SMALL PORTION OF SAID PARABOLIC REFLECTING MEANS WHICH IS DISPLACED FROM ANY EDGE OF SAID PARABOLIC REFLECTING MEANS BY A DISTANCE SUFFICIENT TO SUBSTANTIALLY ELIMINATE EDGE EFFECT; WHEREBY A PLANE WAVE IS RADIATED FROM SAID PARABOLIC REFLECTING MEANS TOWARD SAID ANTENNA BEING TESTED AND THE APERTURE OF SAID PLANE WAVE IS SUBSTANTIALLY EQUAL TO SAID MAXIMUM APERTURE DIMENSION OF SAID ANTENNA BEING TESTED. 